1. Field of the Invention
The invention relates to a voltage converter comprising a first supply terminal, a second supply terminal, an output terminal; a first charge pump comprising a first capacitor with a first and a second electrode, first means for alternately coupling either the first electrode of the first capacitor to the first supply terminal and the second electrode of the first capacitor to the second supply terminal, or the second electrode of the first capacitor to the first supply terminal; a second charge pump comprising a second capacitor with a first electrode and a second electrode, second means for alternately coupling, in phase with the first means, either the first electrode of the second capacitor to the first supply terminal and the second electrode of the second capacitor to the second supply terminal, or the second electrode of the second capacitor to the first supply terminal; and a first unidirectional element coupled between the first electrode of the first capacitor and the output terminal.
2. Discussion of the Related Art
Such a voltage converter is known from European Patent 0,626,750 and serves for generating a DC voltage which is higher than the supply voltage of the voltage converter. The voltage converter may be used, for example, in integrated circuits in cases in which a portion of the integrated circuit requires a DC voltage of a higher value than the value of the supply voltage. The first means comprise a first transistor which is coupled between the first supply terminal and the first electrode of the first capacitor, and a first inverter whose output is coupled to the second electrode of the first capacitor and whose input is coupled to an output of a pulse generator. The second means comprise a second transistor which is coupled between the first supply terminal and the first electrode of the second capacitor, and a second inverter whose output is coupled to the second electrode of the second capacitor and whose input is coupled to the output of the pulse generator. The known voltage converter also comprises a third and a fourth charge pump with third and fourth means. The third and the fourth charge pump and the third and the fourth means are of identical construction as compared with the first and the second charge pump and the first and the second means, respectively. The third and the fourth means operate in counterphase to the first and the second means. During a second phase, the second charge pump supplies a DC voltage to a control electrode of a transistor of the fourth charge pump; during a first phase the fourth charge pump supplies a DC voltage to a control electrode of the second transistor. The first capacitor is charged in the first phase by means of a coupling of the first electrode of the first capacitor to the first supply terminal via the first transistor, and by means of a coupling of the second electrode of the first capacitor to the second supply terminal via the output of the first inverter. The potential at the first electrode of the first capacitor reaches a value which is substantially equal to the potential at the first supply terminal because in the first phase the potential at the control electrode of the first transistor is approximately twice as high as the potential at the first supply terminal, so that the first transistor also remains in the conducting state when the potential at the second electrode of the first transistor approaches the potential at the first supply terminal. In the second phase, the potential at the first electrode of the first capacitor is approximately twice as high as the potential at the first supply terminal owing to a coupling of the second electrode of the first capacitor to the first supply terminal via the output of the first inverter. The first transistor is non-conducting in the second phase. The rectifier diode transfers a quantity of charge from the first capacitor to a load which is coupled to the output terminal. The potential at the first electrode of the first capacitor drops as a result of this, so that a voltage ripple arises at the first electrode of the first capacitor. Since the voltage difference across the rectifier diode is constant by first-order approximation, the voltage ripple at the first electrode of the first capacitor causes a voltage ripple at the output terminal.
It is a disadvantage of the known voltage converter that a smoothing capacitor with a comparatively high capacitance value must be coupled to the output terminal for obtaining an output voltage with a low ripple value.